Spray dryer dust explosion

The spray dryer has been in service for many years and had never had any trouble before. It is used to dry an agricultural product with dust indices in the low 100 bar m/s, and 7 bar range, numbers that are typical for many similar operations. There were no explosion protection devices installed such as vents or suppression. An explosion lifted the roof of the dryer and caused extensive damage to the room and associated equipment.

The investigation found several factors that led to the event. The dust was tested and found to display self-heating behaviour at a temperature in the low 95°C range. The temperature of the gas-heated air in the dryer had been raised from the low to mid 380°C range just a few days before the event. The duration of continuous operation between cleanings had been increased from five days to over two weeks at the time of the explosion. There were other contributing factors as well, but these are the main ones.

The raise in operating temperature resulted in the walls of the dryer becoming hot enough to reach self-heating levels for dust that might accumulate and remain in contact with them. The increase in the operating cycle allowed material to accumulate and dwell long enough for self-heating to result in ignition. Initially, the material was on fire, but the situation quickly deteriorated. Water should have replaced wet slurry, but the spray head was clogged and no water entered the dryer. The loss of wet feed led to a rapid temperature excursion. At this point the process was totally out of control and explosion was inevitable. With no explosion protection system, the dryer sustained extensive damage particularly in the top section.

Recommendations for remediation included installation of either explosion vents or suppression, a sound idea for every spray dryer that is handling combustible dust. Unfortunately, the dryer was not strong enough to make these options easy to implement. Additionally, provisions for changes in process control and instrumentation were undertaken as necessary to try to prevent a recurrence. Taken in total the modifications proved to be extensive, costly and time consuming to complete.

In the meantime no product was being dried and this started to cause serious shortages in the downstream supply chain. From the time of the explosion to the investigation, the design of modifications, consideration of the available options, gathering of bids, awards of orders and installation of new equipment took about three months, which is a remarkably short period of time, all things considered. Nevertheless, time ran out and all of the explosion protection equipment could not be delivered, installed and commissioned in time to satisfy the demand for production.

The management of the company faced a huge dilemma. Further delay would put them out of business. If they operated their spray dryer without the explosion protection systems installed and fully functional, they were putting themselves in harm's way. This was particularly so in light of the loss history associated with this dryer handling this product.

Every spray dryer that is handling combustible dusts should be fitted with explosion protection systems.